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<head>
    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        function fn(num) {
            if ( num == 1 ) {
                return num * 1 ;
            }
            return num * fn( num - 1 );
        }
        var res = fn(5);
        alert(res);
    </script>
</body>
</html>